The present invention relates to the field of mobile radio systems from 2.5 generation, and more precisely to a method for the early establishment of uplink TBFs. To simplify the disclosure, used acronyms are given at the end of the description. For the sake of simplicity only the (E)GPRS embodiment will be described, but the same conclusions are valid for the other 3GPP systems (UMTS)
FIG. 1 shows the functional architecture of a GSM/EDGE network according to TS 44.060 V.6.2.0, including the following functional blocks: SGSN, GGSN, EIR, BSS, MSCNLR, HLR, SMS-GMSC, SMS-IWMSC, SM-SC and MS. The latter includes a first functional block TE connected to a second functional block MT through a connection indicated by a Reference point R, typically supporting a standard serial interface. The MT block is connected to the Um interface supplying packet data services on radio channels. The following interfaces are foreseen: Um, Gi, Gp, Gb, Gn, Gp, Gf, Gs, Gr, Gd, D, E, C, whose connectivity between the relevant blocks are directly visible in the figure. In particular, the Gn interface connects two GSN nodes in the same PLMN system, while the Gp interface connects two GSN nodes belonging to different PLMN systems. The BSS block includes a plurality of BTS connected to a through a respective Abis interface (FIG. 2).
In operation, at the Um and Abis interfaces several protocols are stacked upon the physical layer, in particular MAC, RLC, and LLC. RLC gives a reliable radio link and maps the LLC frames within the physical GSM channel. Signalling procedures for accessing the radio channel are controlled by MAC, which also governs dynamic allocation of the resources (request and grant). Dynamic allocation means that a particular transmission resource, including for instance of a PDCH channel on a physical timeslot, is made time division shareable among more MS mobiles, each of them being engaged in an active session of data transfer, or signalling, through the same transmission resource jointly assigned.
The sub-set of MAC procedures governing the dynamic allocation of resources, provides temporary connections on the physical layer, called TBFs, which include memory buffers to house the queues of RLC/MAC blocks. Each TBF connection enables the unidirectional transfer of data and user signaling within a cell between the network and a mobile station MS, or vice versa. Control messages for the establishment/abatement of a connection and the allocation/de-allocation of relevant supported physical resources, for instance of TBF buffers, contemplate different opportunities capable of covering the whole survey foreseen in the packet transfer mode of the RR sublayer. For simplicity, it is here described a very limited survey of establishment/abatement of TBF connections and of the relevant operation modes. We can start from the establishment of a TBF uplink connection following a Packet Transfer originated by the mobile. In this case the mobile requires the assignment of a GPRS channel sending a PACKET CHANNEL REQUEST message including the resources requested for the transfer of packets to the network. In case of reception, the network replies with a PACKET UPLINK ASSIGNMENT message on the control channel allocating to the mobile the resources requested for the uplink transfer of packets. The resources include one or more PDCH channels and a TFI value. The network does not assign any buffer in uplink direction (the buffer resides in the mobile). The network requires simply knowing the number of blocks that a MS mobile intends to transmit. We can now proceed examining the establishment of a TBF connection downlink following a Packet Transfer ended towards the mobile. In this case at the end of the paging procedure, the network sends the mobile a PACKET DOWNLINK ASSIGNMENT message in the Ready state on the control channel, with enclosed the list of PDCH channels allocated for the downlink transfer. A buffer, relevant to the downlink TBF, is purposely allocated to contain the RLC/MAC blocks to be sent.
In the majority of the cases a TBF is kept alive only for the transfer of one or more LLC protocol units, to the right purpose of transferring the corresponding RLC/MAC blocks. The network assigns each TBF connection its own temporary identifier, called TFI (Temporary Flow Identity). The mobile shall assume that the TFI value is unique among TBF competitors in each direction, uplink or downlink. A RLC/MAC data block is identified to the TBF to which it is associated through its own field where the identifier TFI is written, and another field to indicate the uplink or downlink direction of the block. Should the RLC/MAC block be referred to a control message, a field is foreseen to indicate the message transmission direction and type. In the case of dynamic allocation, the header of each RLC/MAC block transmitted on a PDCH channel in “downlink” direction includes an additional field called USF, which is used by the network in the form of a flag to control the time division multiplexing of different mobile stations on a physical channel PDCH in uplink direction. We can now better qualify the already mentioned PACKET UPLINK ASSIGNMENT message, sent by the network towards the mobiles, stating that it includes: the identifier TFI of the downlink/TBF buffer containing the control block carrying this message, the list of the allocated PDCH channels (time slots), and a corresponding USF value for each allocated channel. Three bits are foreseen for the USF field that enable to unambiguously discriminate up to eight users sharing a time-slot, also in the borderline case in which the single TBF buffer are associated all the eight time slots of a TDMA frame.
Outlined Technical Problem
The setup time of a TBF in (E)GPRS system might be in the order of several hundreds of milliseconds. In the case of an uplink TBF this is due to the combined effects of the random access procedure, the bring-up time on the A-bis interface, the round-trip time between the mobile station and the PCU, etc.
For delay sensitive real-time applications a setup time of hundreds of milliseconds would impair the performance greatly. For instance, this is the case for applications, or VoIP over cellular.
In 3GPP, TSG GERAN is currently discussing possible enhancements to reduce latency in the PS domain, including solutions to reduce the initial setup time of a TBF.
One idea, which has already been discussed in standardization, is the possibility to enable an “early TBF establishment”, i.e. the possibility for a mobile station to request an uplink TBF even before some actual data is ready for transmission.
If a mobile station is enabled to open an uplink TBF in advance (i.e. before some actual data is available for transmission), this means that no setup time will be needed as soon as there is a real need to transmit something in the uplink.
Some specific proposals on how to realize this behaviour have already been presented for standardization: see for instance GP-052038 and GP-052039 (respectively become G2-050396 and G2-050397) on “Phantom TBFs”. But such proposals have some important drawbacks. For instance:                they foresee the definition a special TBF type, only needed for this procedure, adding extra complexity in both the MS and the network;        they are linked to the support of multiple TBFs in both the MS and in the network, thus limiting the general applicability of the solution;        they are linked to the early detection of a SIP signalling indication by the MS.        